Recently, in batteries such as lithium ion secondary batteries and nickel-hydrogen secondary batteries, improvement in the energy density has shown marked progress, and batteries have driven equipment for a long time while maintaining miniaturization and lightness. The performance improvement of batteries contributes largely to the supply of portable electronic equipment represented by portable phones.
Also, along with the high performance of the battery body, the performance of peripheral circuits has also advanced. For example, a battery device (also called a battery pack) in which an electric circuit for realizing various functions such as residual amount management function is assembled into the battery body, can be mounted in electronic equipment such as notebook type computers and video cameras in which batteries can be exchanged. Recently, battery devices for realizing these functions using a microcomputer (hereinafter, sometimes called “μC”) have been typical.
On the other hand, for example, in high-performance batteries such as lithium ion secondary batteries, if the cell voltage is abnormally raised by an overcharge, or the cell voltage is extremely lowered by an overdischarge, or if excessive charge and discharge current flows in cells, a degradation in characteristics and problems are apt to be structurally caused. For this reason, a circuit for protecting batteries by cutting off the power supply path between the batteries and electronic equipment, if an abnormal charge and discharge is caused, is mounted in typical battery devices. In Japanese Kokai Patent Application No. 2005-160169, a technique pertaining to mounting a battery protecting circuit in a battery device is described.
On the other hand, in this battery device, in order to operate a built-in microcomputer and battery protecting circuit, a power supply circuit that steps up and down the voltage of the battery and generates a power supply voltage at a fixed level is installed.
For example, in equipment such as portable phones in which the voltage of the battery is relatively low and the power capacity is also small, a switching regulator is sometime employed as a power supply circuit. However, in equipment using a high-capacity and high-voltage battery, since noise being generated by a switching regulator is likely to influence the operation of the equipment, it is typical to use a low dropout type linear regulator (hereinafter, sometimes abbreviated to LDO) with low noise.
On the other hand, if the number of cells connected in series increases, the capacity of the battery can be raised accordingly, but since the loss generated in the above-mentioned LDO is increased, the increase in the number of cells is disadvantageous in terms of low power consumption. Also, if devices such as light-emitting diodes and flash memories that require a relatively large current consumption are present, a radiation for the LDO is sometimes required, and for this purpose, if a radiation means such as radiating plate and heat sink is installed, it is disadvantageous in terms of miniaturization and high densification of the equipment.
A general object of the present invention is to provide a power supply circuit with low noise and small power consumption and a battery device using the power supply circuit.